The present invention relates to devices and methods for thawing frozen materials by exposing same to electromagnetic energy, and more particularly to such devices and methods wherein thawed liquid is removed from exposure to the energy to prevent overheating the liquid.
Many heat sensitive materials are frozen to prolong storage life. These include foodstuffs, pharmaceuticals, and particularly blood and blood products. It is often desirable to thaw these materials quickly, especially blood needed in emergency situations. At the same time, it is well known that it is very difficult to thaw frozen materials by microwave heating in a controlled and reproducible way, because the loss tangent of water is so much greater than that of ice. Once a small portion of the material is melted, that portion rapidly absorbs additional microwave energy and begins cooking.
In the field of microwave radiation, it is well known that microwave ovens may be constructed to operate at either fixed or variable frequency. Owing to the coupling ability of 2.45 GHz microwaves to water, this frequency is often used for cooking foods, drying, and other purposes wherein the principal material to be acted upon is water. Most commercial units operate at frequency range of 2.45 GHz +/xe2x88x9225 MHz, and some as hi +/xe2x88x9250 However, it is well known that a multimode cavity operating at fixed frequency will display significant nonuniformities in the spatial power density owing to the formation of standing waves (or the excitation of only a small number of microwave modes within the cavity).
Recently, the use of frequency sweeping over a wide range as a means of mode stirring has been demonstrated and patented (Bible et al., U.S. Pat. No. 5,321,222). Modeling results and experimentation have shown that for typical multimode applicator cavities a bandwidth of about +/xe2x88x925% of a center frequency provides a relatively uniform power density because of the superposition of many independent microwave modes (Bible et al. U.S. Pat. No. 5,961,871). Electronic frequency sweeping may be performed at a high rate of speed, thereby creating a much more uniform time-averaged power density throughout the furnace cavity. The desired frequency sweeping may be accomplished through the use of a variety of microwave electron devices. A helix traveling wave tube (TWT), for example, allows the sweeping to cover a broad bandwidth (e.g., 2 to 8 GHz) compared to devices such as the voltage tunable magnetron (2.45+0.05 GHz). Other devices such as klystrons and gyrotrons have other characteristic bandwidths, which may be suitable for some applications.
In fixed frequency ovens, attempts have been made at mode stirring, or randomly deflecting the microwave xe2x80x9cbeamxe2x80x9d, in order to break up the standing modes and thereby fill the cavity with the microwave radiation. One such attempt is the addition of rotating fan blades at the beam entrance of the cavity (Mizutani et al. U.S. Pat. No. 4,629,849). Alternatively, rotating feed horns (Kaneko et al. U.S. Pat. No. 4,176,266) and multiple feed horns (Jurgensen U.S. Pat. No. 3,916,137) have been described. None of these approaches creates a substantially uniform microwave power density within a xe2x80x9csmallxe2x80x9d multimode cavity. Mechanical mode stirring devices do not in general provide enough of a physical perturbation and there is a limit to how fast they can be moved. Using multiple feeds becomes impractical when the number of feeds exceeds more than a few, and this is generally not adequate for true power uniformity within the cavity.
Another method used to overcome the adverse effects of standing waves is to intentionally create a standing wave within a single-mode cavity such that the workpiece may be placed at the location determined to have the highest power (the hot spot). Thus, only that portion of the cavity in which the standing wave is most concentrated will be used.
Other devices have been produced to change the parameters of the heating process of selected materials. Typical of the art are those devices disclosed in the following U.S. Patent:
As previously mentioned, Bible et al. have described how frequency sweeping over a selected bandwidth, typically 5%, could establish a substantially uniform microwave power distribution within the cavity by the superposition of many hundreds of microwave modes. Nevertheless, none of the aforementioned approaches can completely address the fundamental difficulty of microwave thawing, namely, the large difference in dielectric loss between water and ice. The large increase in loss tangent upon melting creates an inherently unstable heating process in which the first volume of material to melt begins to absorb power selectively, rapidly leading to localized thermal runaway.
Accordingly, it is therefore an object of this invention to provide a microwave or other electromagnetic energy heating apparatus in which a frozen material may be subjected to a controlled application of the energy.
It is another object of the present invention to provide a microwave or other electromagnetic energy heating apparatus in which one may control the absorption of the energy within a frozen material to selectively begin melting the material at predetermined areas.
It is another object of the present invention to provide a microwave or other electromagnetic energy heating apparatus in which one may protect already-melted liquid from further exposure to the energy by providing a shielded region for the thawed liquid.
It is a further object of the present invention to provide a microwave or other electromagnetic energy heating apparatus in which one can manage the flow of liquid after melting to prevent the entrapment of liquid in areas that are exposed to the energy.
It is yet another object of the present invention to provide a method of applying a controlled concentration of microwave or other electromagnetic energy to a container of frozen material.
It is another object of the present invention to provide a method of controlling the absorption of microwave or other electromagnetic energy within a frozen material to selectively begin melting the material at predetermined areas.
Yet another object of the present invention is to provide a method of thawing in which already-melted liquid is protected from further exposure to microwave or other electromagnetic energy.
It is a further object of the present invention to provide a method for thawing in which the flow of liquid after melting is controlled to prevent the entrapment of liquid in areas that are exposed to microwave or other electromagnetic energy.
Further and other objects of the present invention will become apparent from the description contained herein.
In accordance with one aspect of the present invention, the foregoing and other objects are achieved by an apparatus for thawing a frozen material. The apparatus includes: an electromagnetic energy source; an energy applicator which defines a cavity for applying microwave energy from the microwave source to a material to be thawed; and a shielded region which is shielded from the energy source, the shielded region in fluid communication with the cavity so that thawed material may flow from the cavity into the shielded region.
In accordance with another aspect of the present invention, an apparatus for thawing of selected materials includes: a multimode microwave applicator cavity; a microwave source adapted for sweeping the frequency of microwave energy introduced into the cavity over a usable bandwidth of at least +/xe2x88x922% of a center frequency so that the microwave power density within the cavity is substantially uniform; and a shielded region which is shielded from the microwave source, the shielded region in fluid communication with the cavity so that thawed material may flow from the cavity into the shielded region.
In accordance with a further aspect of the present invention, a method of thawing selected materials includes the steps of: providing an electromagnetic energy source; an energy applicator which defines a cavity for applying energy from the energy source to a material to be thawed; and a shielded region which is shielded from the energy source, the shielded region in fluid communication with the cavity so that thawed material may flow from the cavity into the shielded region; placing a material to be thawed into the microwave applicator cavity; and introducing microwave energy into the applicator cavity to thaw the material so that thawed liquid flows from the cavity into the shielded region.
In accordance with another aspect of the present invention, a method for microwave-assisted thawing of selected materials includes the steps of: providing a multimode microwave applicator cavity; a microwave source adapted for sweeping the frequency of microwave energy introduced into the cavity over a usable bandwidth of at least +/xe2x88x922% of a center frequency so that the microwave power density within the cavity is substantially uniform; and a shielded region which is shielded from the microwave source, the shielded region in fluid communication with the cavity so that thawed material may flow from the cavity into the shielded region; placing a material to be thawed into the microwave applicator cavity; and introducing microwave energy into the applicator cavity to thaw the material so that thawed liquid flows from the cavity into the shielded region.